The present invention relates to methods of making friction materials, and more particularly friction materials for a friction crown, or crown shaped friction liner, for a clutch friction wheel or friction disc, designed for dry operation. The invention equally relates to such a friction crown itself.
A friction crown of the above type is made from a yarn which is based in particular on mineral fibres such as glass fibre. This provides mechanical strength against the centrifugal force which occurs during operation of the friction crown. The basis of the friction crown material also includes rubber in order to give a good coefficient of friction, and includes various fillers and a binder, which in practice is a phenolic resin, in order to provide cohesion between the various constituents.
During manufacture of the crown, it is usual to make use of various solvents, to dissolve the rubber. These solvents are chlorinated solvents which disadvantageously are noxious and consequently make it necessary to perform operations to confine the process within the production line and to recover the resulting vapours, to prevent any contact between the operators working on the manufacture and any release of noxious vapours into the atmosphere.
It has previously been proposed to make use of aqueous solvents to replace the chlorinated solvents. This does however make it necessary to use latex as well as rubber. In practice, an aqueous cement is made which results from the mixture of phenolic resins, fillers and latex in water. This cement is then used for impregnating a yarn consisting of mineral and other fibres, which serves for the formation of a blank. The blank is subsequently heat treated under pressure so as to form the friction crown itself.
This aqueous cement has a viscosity which increases very rapidly over time. As a result firstly, only a very short time is available during which the cement is able to be used for impregnation purposes; and secondly, there are undesirable variations in the friction and wear resistance characteristics of the friction crowns which are then made.
An object of the present invention is to provide a method of making a friction material, and more particularly a friction material for a friction crown for a dry friction clutch disc, which incorporates an aqueous cement that does not have the above mentioned drawbacks.
According to the invention in a first aspect, a method of making a friction material, the method includes an operation of impregnating with an aqueous impregnating cement a yarn based, on mineral fibres such as glass fibres, such that the aqueous cement contains latex together with a phenolic resin which is at least partly in the form of a phenolate.
The phenolate is preferably an alkaline phenolate. To this end, a strongly basic constituent (i.e. an alkali or other chemical base) is preferably added to the impregnating cement, before or after addition of phenolic resin.
The phenolic resin is preferably in the form of a powder.
The strongly basic constituent may for example be potash or soda.
Preferably, the proportion by weight of the strongly basic constituent, with respect to the phenolic resin, is in the range between 6 and 15%, and is preferably between 8 and 15%.
In a cement according to the invention, the length of time during which the cement is able to be used for impregnating the yarn so as to form the skeleton and blank of the friction crown, is considerably increased. In practice that time is in fact multiplied by a factor of the order of 10.
According to a feature of the invention, the manufacture of the impregnating cement includes the formation of a preliminary mixture containing fillers, the phenolic resin, the strongly basic constituent, and water, the method further including the addition of latex to the said preliminary mixture.
Besides the advantages noted above, stability has been observed in the physico-chemical characteristics of the cement during the whole of the time in which it is used, resulting advantageously in, an improved and constant quality in the final product, the friction crown itself.
In-depth analysis has revealed that with the methods conventionally used up to the present time, the addition of phenolic resin as a powder to the mixture containing latex causes water to be absorbed out of the latex. This leads to rapid increase in viscosity, and makes the cement unsuitable for impregnation purposes.
The use of resin in the form of a phenolate, or the addition of a strong base which is able to transform at least part of the resin into phenolate in accordance with the invention, avoids the above mentioned problems encountered in current practice.
According to the invention in a second aspect, a friction crown for a dry friction clutch disc it comprises yarn based on mineral fibres, such as glass fibres, together with organic fibres such as fibres of polyacronitryl or derivatives thereof and, an aqueous cement comprising a resin in the form of a phenolate, fillers, and latex. This friction crown is preferably made by a method according to the invention.
Further features and advantages of the invention will appear more clearly on a reading of the following detailed description of some embodiments of the method according to the invention, and the product produced by such methods, all of which are given by way of non-limiting example only. The description is further illustrated in the accompanying drawings.